The scanning tunneling microscope, hereinafter STM, is an instrument capable of resolving surface detail down to the atomic level. The microscope's conductive tip, ideally terminating in a single atom, traces the contours of a surface with atomic resolution. The tip is maneuvered to within a nanometer or so of the surface of a conducting substrate so that the electron clouds of the atom at the probe tip overlap that of the nearest atom of the sample. When a small voltage is applied, electrons tunnel across the gap between the microscope tip and the substrate, generating a tunneling current the magnitude of which is sensitive to the size of the gap. Typically the tunneling current decreases by a factor of 10 each time the gap is widened by 0.1 nanometer.
Movement of the microscope tip is controlled by piezoelectric controls. In one mode of operation, the tip or probe is held at a constant height as it is moved horizontally back and forth across the sample surface in a raster pattern, its parallel tracks separated by a fraction of a nanometer. This causes the tunneling current to fluctuate and the current variation is measured and translated into an image of the surface. The current increases when the tip is closer to the surface, as when passing over bumps such as a surface atom, and decreases when the tip is farther from the surface, as when passing over gaps between atoms. In an alternative mode of operation, the probe or tip moves up and down in concert with the surface topography as it is moved across the surface in a raster pattern. Its height is controlled to maintain a constant tunneling current between the tip and the surface. The variations in voltage required to maintain this constant gap are electronically translated into an image of surface relief.
The image obtained by either mode of operation is not necessarily a topographical map of the surface, but a surface of constant tunneling probability affected by the variations in the occurrence and energy levels of the electrons present in the surface atoms. If the surface is composed of a single type of atom, the image may closely resemble topography, but when various atoms are present pits or bumps will appear in the image depending upon their electronic structures.
Further detail on the structure and operation of the STM is disclosed in the Binnig et al. U.S. Pat. No. 4,343,993 of issued Aug. 10, 1982; Wickramasinghe, H. K., Scientific American, 98-105, October (1989); and Hansma et al., Science, 242, 209-216, Oct. 14, 1988.
The STM is useful not only for the imaging or characterization of surfaces, but also for manipulating surfaces at the subnanometer scale. Lithography using STM for nanoscale structure fabrication is of interest in the area of electronics for information storage bits, nanoelectronic circuit elements, and other applications in microelectronics. The ability to manipulate single atoms or molecules with the STM provides many unique potential applications in microelectronics.
Various approaches have been explored in the use of STM for etching or writing. Since STM is limited to imaging or manipulating surfaces which conduct electrons, thin conductive coatings or replicas have been used on substrate surfaces which are nonconducting. Metal deposition onto a substrate surface from a gas is another method which has been used to pattern lines using the STM. Deposition of particles onto the surface from a carrier has also been used. The formation of protrusions or raised surface areas on metallic glasses by local heating is another STM writing technique. Writing using the STM wherein the microscope tip physically touches, scratches, indents or creates holes in the substrate surface is also known. The tunneling current has also been used for surface rearrangement of atoms already present.
One popular method for generating nanometer scale structures with the STM involves using a short voltage pulse of nanoseconds to microseconds duration and a few volts in amplitude. Variations of such methods can be found in the published literature such as Forster et al., Nature, 133, p. 324, Jan. 28, 1988 and Miller et al., J. Appl Phys., 68, p. 905 (1990).
It is known that such nanometer scale structures can be made under water as described by Penner et al., Abstracts for STM 1990/Nano I Conference, July 1990. Such structures have also been made in a previously evacuated chamber partially filled with water vapor as disclosed by Albrecht et al., Appl Phys. Lett., 55, p. 1727, (1989).
However there are still problems with the methodology. In some operating environments the conditions of the voltage pulse for making the structures seem to change slightly after each pulse. Typically the voltage amplitude has to be turned up one or two tenths of a volt after each pulse, thus making automation difficult. Also the reliability of the fabrication process appears to be deficient. Using identical procedures for the voltage pulse and substrate preparation, the size of the structures can change drastically. It can be impossible to predict whether a structure will appear.
It is therefore an object of the present invention to provide an improved process for nanometer scale fabrication using STM.
It is a further object of the present invention to provide a process for etching or writing which is reliably reproducible.
It is a further object of the present invention to provide a reproducible fabrication process by which structures of less than 10 nanometers can easily be prepared.
It is a further object of the present invention to provide a process for retaining moisture in a substrate of the type for nanometer scale fabrication in a vacuum.